docs: def _index_entities(self):
          ''' Sets current instance's entities based on the existing index.
  
          Note: Only entity key/value pairs common to all rows in all contained
              Variables are returned. E.g., if a Collection contains Variables
              extracted from runs 1, 2 and 3 from subject '01', the returned dict
              will be {'subject': '01'}; the runs will be excluded as they vary
              across the Collection contents.
          '''
          all_ents = pd.DataFrame.from_records(
              [v.entities for v in self.variables.values()])
          constant = all_ents.apply(lambda x: x.nunique() == 1)
          if constant.empty:
              self.entities = {}
          else:
              keep = all_ents.columns[constant]
              ents = {k: all_ents[k].dropna().iloc[0] for k in keep}
              self.entities = {k: v for k, v in ents.items() if pd.notnull(v)}